Because in German jurisdiction, linking to illegal content can bring with it fines. It’s ridiculous, but that’s where we are, more specifically we have a lawyer who is insanely prolific at sending out legal threats and asking for ridiculous amounts of money for this.
And now the problem: Due to the way ActivityPub works, one instance “pulls” the links posted on another instance to itself. At that point, lemmy.world is hosting links to content XYZ, not the other instance. Hence they’d be legally actionable. And being the largest instance of lemmy, this is the one any lawyer firm wanting to make quick bucks or any music industry wanting to appear like they’re doing anything worth all the money to their consitutents would go after this one, not any other instance.
The protocol is a bit of a double-edged sword: Yes you’re not hosting the content, but if links can be fined then those get pulled over essentially without you being able to do shit about it.